The invention relates to an indicator circuit for providing an indication of the operation of an auto-strobo unit, and more particularly, to an improved indicator circuit which indicates whether a flash discharge tube has automatically operated to provide a proper amount of flashlight illumination in response to the closure of synchro contacts provided on the part of a camera.
An indicator which indicates the operation of an auto-strobo unit generally comprises a capacitor which is charged by a portion of the discharge current from a main capacitor connected in the discharge circuit of the strobo unit and which discharges through a neon discharge tube representing an indicator tube. However, once charged, it requires a finite length of time, which may be on the order of three seconds, for example, for the charge on the capacitor to be discharged through the neon discharge tube to extinguish its illumination. Hence, if pictures are taken in rapid succession using the flashlight illumination, the indicator charging capacitor becomes charged anew before it is completely discharged, producing a continued display. This presents no problem whatsoever so long as the illumination produced by the strobo unit is sufficient to assure a proper photographing operation. However, in the event the main capacitor is insufficiently charged to provide a full exposure by the emission of the flashlight or to cause such emission at the time the synchro contacts are closed in response to a shutter release, the neon discharge tube is energized by the discharge current from the associated capacitor which remains charged from the previous strobo operation despite the fact that the exposure level decision circuit of the strobo unit does not produce a full exposure signal. This results in a misleading recognition on the part of the user in that he assumes that a proper exposure has ocurred by seeing the illumination of the neon discharge tube when the film is subject to an under-exposure, which is revealed only upon developing the film.
Before proceeding with the description of the invention, the above disadvantage will be described more fully with reference to FIG. 1 which shows a conventional indicator circuit associated with an automatically controlled strobo unit. In FIG. 1, there is shown a high voltage source 1 across which a main capacitor 2 is connected in series with diode 10. A discharge illumination circuit including trigger transformer 3 as well as a series combination of flash discharge tube 4 and main switching element 5 is connected across the capacitor. Connected across the main switching element 5 are a commutation circuit including capacitor 6 and switching element 7 as well as an indicator circuit including capacitor 8 and display element 9.
Specifically, the capacitor 2 has its positive electrode connected with the cathode of the diode 10, with the junction therebetween being connected with a positive bus L. A negative bus E.sub.O is connected with the negative terminal of the capacitor 2. The trigger transformer 3 includes a primary coil 3a which is connected through trigger capacitor 12 and resistor 11 with the bus L, and also includes a secondary coil 3b which has its one end connected with the trigger electrode 4a of the flash discharge tube 4. Synchro contacts 13 are connected in shunt with the series combination of trigger capacitor 12 and primary coil 3a, and are adapted to be closed in response to a shutter release operation of a camera. Both the primary and secondary coils 3a, 3b are connected together at junction a, which is connected with the gate of the main switching element 5, comprising a thyristor, through resistor 14.
The main switching element 5 is connected in series with the flash discharge tube 4 and with a parallel combination of coil 15 and diode 16 across the main capacitor 2, and has its gate connected with the bus E.sub.O through resistor 17. The series combination of the tube 4 and main switching element 5 is shunted by a series circuit including resistor 18 and commutating switching element 7 which is a thyristor. The gate of the element 7 is connected with the bus E.sub.O through resistor 20, and is adapted to receive an exposure complete signal from an exposure level decision circuit 19 of known form which may include a photometric circuit.
The commutating capacitor 6 is connected between the anode of the main switching element 5 and that of the commutating switching element 7. A series combination of diode 21 and indicator operating capacitor 8 is connected in shunt with the main switching element 5. The display element 9 and capacitor 23 are connected in shunt with the capacitor 8 through resistor 22. A resistor 24 is connected in shunt with the main switching element 5, and normally cooperates with resistor 18 to charge the commutating capacitor 6 to the polarity shown in circles, its electrode which is connected with the switching element 7 being charged to the positive polarity.
In operation, when a shutter release of a camera is actuated to close the synchro contacts 13 to provide a flashlight photographing operation assuming that the main capacitor 2 is sufficiently charged, the combination of the trigger capacitor 12 and trigger transformer 3 triggers both the flash discharge tube 4 and main switching element 5, whereby the charge on the main capacitor 2 discharges through both of these elements, thus producing a synchronized flashlight illumination. When a proper amount of exposure has been given, the exposure level decision block 19 including a photometric circuit is activated to provide an exposure complete signal which is applied to the gate of the commutating switching element 7. Thereupon, the element 7 is turned on, whereby the commutating capacitor 6 discharges through the switching elements 5 and 7, thus passing a reverse current flow through the main switching element 5 to turn it off. A discharge current continues to flow through the flash discharge tube 4 after the element 5 is turned off, the discharge current passing through the tube 4, commutating capacitor 6 and commutating switching element 7 to charge the capacitor 6 to the opposite polarity from that shown in circles. It will then be noted that though the electrode of the capacitor 6 connected with the resistor 24 momentarily assumes a negative potential relative to the bus E.sub.O at the time the element 7 is turned on, it immediately reverts to a positive polarity. As the discharge current passing through the tube 4 gradually decreases and becomes less than a holding current, the tube 4 completely ceases to illuminate, thus completing an automatic illumination operation which provided a proper exposure. When the electrode of the capacitor 6 connected with resistor 24 reverts to a positive potential, the diode 21 is forwardly biased to conduct part of the discharge current passing through the tube 4, thus charging the capacitor 8. As the capacitor 8 is charged, it begins to charge capacitor 23 through resistor 22. When the voltage across the capacitor 23 reaches the threshold voltage of the display element 9 which may, for example, be a neon discharge tube, the charge on the capacitor 23 discharges through the element 9, thus illuminating it to provide an indication that a proper exposure has been given.
It is to be understood that the neon discharge tube which is used as the display element 9 has a hysteresis phenomenon, so that the element 9 continues to illuminate until the voltage across the capacitor 23 decreases below an extinction voltage of the element 9. After extinction of the element 9, the capacitor 23 begins to charge again through resistor 22 until the voltage thereacross reaches the threshold voltage of the element 9, whereupon the latter is again illuminated. The indicator circuit comprising capacitors 8, 23, resistor 22 and element 9 repeats such operation, and hence the display element 9 continues its flashing operation with a suitable time constant until the voltage across the capacitor 8 reduces below the threshold voltage of the element 9. When the strobo unit operates to provide a flashlight illumination and to charge the capacitor 8, the indicator circuit effects a flashing operation for a given time interval independently from a subsequent operation of the strobo unit. This display is designed to continue for a time interval on the order of three to five seconds in order to permit a user to recognize the operation of the strobo unit when he moves his eyes from the viewfinder after he has actuated a shutter release while viewing the viewfinder of a camera.
However, it is to be noted that if the indicator circuit effects a flashing operation in the event the main capacitor 2 is insufficiently charged to provide a proper exposure or to activate the flash discharge tube 4 when pictures are taken in rapid succession using the flashlight illumination, as in an automatic continuous photographing operation using a motor drive, the user is misled that a proper exposure has been given inasmuch as the indicator circuit fails to indicate an underexposure.